Laser treatment is increasingly being used for both experimental and clinical medical purposes. For example, lasers are often employed for photocoagulation or to treat heart ailments, various cancers, etc. The success of such laser treatments depend upon the manner in which the light emitted by the laser and the tissue interact, such as by heating the tissue. As explained in Splinter, et al., "Tissue Diagnostics by Depth Profiling of Optical Characteristics Using Broadband Fiber-Optic Interferometry", Coherence-Domain Methods in Biomedical Optics, SPIE Vol. 2732, pp. 242-50 (August 1996), the manner in which the light emitted by the laser interacts with the tissue depends, in large part, upon the optical properties, such as the absorption and scattering coefficients, of the tissue at the particular wavelength of light with which the tissue is illuminated. In order for subsurface laser treatments to be effective, an optical window must be defined based upon the light-tissue interaction which identifies one or more wavelengths of light for which the tissue has a relatively high optical penetration, thereby efficiently providing subsurface tissues with the required dosage of light to cause the desired photochemical reaction, such as photocoagulation or other photochemical reactions.
In most instances, biological tissues are highly scattering with the particular optical properties, i.e., the scattering and absorption properties, depending on the tissue and the pathological condition of the tissue at the irradiation wavelength. Color and composition of the tissue to be treated can sometimes provide evidence of the optical and thermal properties of the tissue. However, while the optical properties differ for different tissues and for different pathological tissue conditions, these different optical properties are not always distinguishable by visual examination of the tissue.
Since tissue responds differently to light of different wavelengths, the various medical procedures which employ laser illumination can be enhanced by properly identifying the tissue and by thereafter selecting the proper wavelength of light for optimally treating the tissue. The tissue can then be illuminated by light of the proper wavelength to effectively treat the tissue to desired depths.
To more accurately identify tissue samples, a variety of optical coherence tomography techniques have been developed. For example, U.S. Pat. No. 5,459,570 to Swanson, et al., and an article entitled "Optical Coherence Tomography" by Huang et al. which appeared in Science, Reprint Series, Vol. 254, pp. 1178-81 (Nov. 22, 1991), describe optical coherence domain reflectometers for generating images of tissue samples. As described by these references, an optical coherence domain reflectometer includes an optical source, a reference arm and a sample arm. The optical source may be a super luminescent diode having a short coherence length which emits light that is coupled to the reference arm and the sample arm of the reflectometer. The tissue sample is placed in the sample arm to scatter and reflect the light propagating therethrough. In contrast, a reference mirror is placed in the reference arm to reflect the light transmitted therethrough. The reference mirror can be translated or moved, such as with a stepper motor, in order to vary the length of the reference arm.
The optical coherence domain reflectometers described by these references also include a detector for receiving the signals returned by the reference and signal arms, including the signals which have been reflected and scattered by the tissue sample and the light reflected from the reference mirror. Based upon the interference pattern created by returning signals, the optical coherence domain reflectometers can identify different layers or boundaries within the tissue. See also U.S. Pat. No. 5,321,501 to Swanson, et al., which describes another optical coherence domain reflectometer.
Nonetheless, optical coherence tomography techniques suffer from several deficiencies. For example, optical coherence tomography techniques typically analyze reflections from boundary layers, such as the boundary layer between two different types of tissue. As a result, optical coherence tomography techniques do not generally analyze signals reflected and/or scattered from within a homogeneous tissue layer which may require the subsequent laser treatment. In addition, optical coherence tomography techniques commonly illuminate a tissue sample with light of a single wavelength or a single band of wavelengths. Thus, optical coherence tomography techniques do not typically determine variations in the optical properties of the tissue sample as the tissue sample is illuminated by light of different wavelengths.
As further explained in Splinter, et al., "Monitoring Tissue Optical Characteristics in-situ using a CCD Camera", Lasers in the Life Sciences, Vol. 6(1), pp. 15-25 (1994), the response of biological media or tissue to illumination can be compared to computer simulations, such as Monte Carlo simulations, to identify not only the tissue itself, but also the pathological condition of the tissue. For example, the Splinter article describes a method and an associated apparatus for determining the optical characteristics of a tissue sample in situ so as to thereby identify the ideal laser dosimetry for tissue photocoagulation. According to this method, a laser beam is delivered to a tissue sample by an optical fiber. A CCD camera is also trained on the illuminated tissue sample so as to detect backscattered radiation. The detected backscatter can then be compared to Monte Carlo computer models which simulate backscatter from tissue samples having known optical parameters in order to determine the optical characteristics of the tissue sample. While the technique described by this Splinter article can identify a number of different types of tissue, modern laser treatments are demanding even greater accuracy with respect to the identification of the optical properties of a tissue sample upon illumination with light of different wavelengths and at different depths within the tissue sample.